Tribo-addressed and tribo-supperessed electric paper

ABSTRACT

A display is provided with an insulative layer with electrical properties which have been selected so that the display can be addressed with a stylus and which minimizes the effects of stray triboelectrically generated charges. Also provided is a method of addressing such a display by depositing charges on a surface of the display, maintaining sufficient charge to effect an image change, and then removing the charges.

RELATED APPLICATIONS

This patent application is related to U.S. patent application Ser. No.09/449,019, titled “Tribo-Addressed and Tribo-Suppressed ElectricPaper”, by Mikkelsen et al.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to the field of visual displays. Moreparticularly, the invention relates to a gyricon or twisting rotatableelement visual display having a controlled response to triboelectriccharge effects.

2. Description of Related Art

Paper has traditionally been a preferred medium for the presentation anddisplay of text and images. Paper has several characteristics that makeit a desirable display medium, including the fact that it islightweight, thin, portable, flexible, foldable, high-contrast,low-cost, relatively permanent, and readily configured into a myriad ofshapes. It can maintain its displayed images without using anyelectricity. Paper can also be read in ambient light and can be writtenor marked upon with a pen, pencil paintbrush, or any number of otherimplements, including a computer printer.

Unfortunately, paper is not well suited for real-time display purposes.Real-time imagery from computer, video, or other sources cannot bedisplayed directly with paper, but must be displayed by other means,such as by a cathode-ray tube (CRT) display or a liquid-crystal display(LCD). However, real-time display media lack many of the desirablequalities of paper, such as physical flexibility and stable retention ofthe displayed image in the absence of an electric power source. Electricpaper combines the desirable qualities of paper with those of real-timedisplay media. Like ordinary paper, electric paper can be written anderased, can be read in ambient light and can retain imposed informationin the absence of an electric field or other external retaining force.Also like ordinary paper, electric paper can be made in the form of alight-weight, flexible, durable sheet that can be folded or rolled intoa tubular form about any axis and placed into a shirt or coat pocket,and then later retrieved, re-straightened, and read without loss ofinformation. Yet unlike ordinary paper, electric paper can be used todisplay full-motion and other real-time imagery as well as still imagesand text. Thus, electric paper can be used in a computer system displayscreen or a television.

The gyricon, also called the twisting ball display, rotary ball display,particle display, dipolar particle light valve, rotating elementdisplay, etc., provides a technology for making electric paper. Agyricon display is made up of a multiplicity of optically anisotropicrotatable elements, which can be selectively rotated to present adesired surface to an observer. Additionally, a gyricon display can beaddressed similarly to other displays to present any desired image orsequence of images.

The optical anisotropy of the gyricon rotatable elements may be providedby dividing each gyricon rotatable element into at least two visibleportions. For instance, one portion of the surface of each gyriconrotatable element will have a first light reflectance or color. If onlytwo portions are used then the other portion of the surface of thegyricon rotatable element has a different color or a different lightreflectance. One example of this is a gyricon rotatable element which isspherically shaped and has two distinct hemispheres, one black and theother white. Additionally, each light reflectance characteristic orcolor will have a distinct electrical characteristic, e.g., a zetapotential with respect to a dielectric fluid. Accordingly, the gyriconrotatable elements are electrically as well as optically anisotropic. Itis conventionally known that when particles are dispersed in adielectric liquid, the particles acquire an electric charge related tothe zeta potential of their surfaces.

The black-and-white gyricon rotatable elements are embedded in a sheetof optically transparent material, such as an elastomer layer, thatcontains a multiplicity of cavities. Each of the cavities is permeatedby a transparent dielectric fluid, such as a plasticizer. Thefluid-filled cavities accommodate the gyricon rotatable elements, onegyricon rotatable element per cavity, to prevent the rotatable elementsfrom migrating within the sheet. Each cavity is slightly larger than thesize of the gyricon rotatable element so that each gyricon rotatableelement can rotate or move slightly within its cavity.

A gyricon rotatable element can be selectively rotated within itsrespective fluid-filled cavity by applying an electric field, so thateither a specific portion of the gyricon rotatable element is exposed toan observer viewing the surface of the sheet. By applying an electricfield in two dimensions, for example, using a matrix addressing scheme,the black and white sides of the rotatable elements for instance can becaused to appear as the image elements, e.g., pixels or subpixels, of adisplayed image.

Gyricon displays are described further in U.S. Pat. No. 5,389,945 toSheridon, incorporated herein by reference in its entirety. The '945patent discloses that gyricon displays can be made that have many of thedesirable qualities of paper, such as flexibility and stable retentionof a displayed image in the absence of power, that are not found inCRTs, LCDs, or other conventional display media. Gyricon displays canalso be made that are not paper-like, for example, in the form of rigiddisplay screens for flat-panel displays. Other examples of Gyricondisplays are described in U.S. Pat. No. 5,717,514 titled “PolychromalSegmented Balls For A Twisting Balls Display” by Sheridon issued Feb.10, 1998, U.S. Pat. No. 5,754,332 titled “Monolayer Gyricon Display” byCrowley issued May 19, 1998, U.S. Pat. No. 5,604,027 “Some Uses OfMicroencapsulation For Electric Paper” by Sheridon issued Feb. 18, 1997,U.S. patent application Ser. No. 08/716,672 titled “Twisting CylinderDisplay” by Crowley and Sheridon, filed Sep. 13, 1996, and U.S. Pat. No.5,894,367 titled “Twisting Cylinder Display Using Multiple ChromaticValues” by Sheridon, issued Apr. 13, 1999. These describe manyvariations of gyricon displays including monolayer construction,cylindrical rotating elements, and rotating elements constructed todisplay more than two colors or constructed as light valves.

SUMMARY OF THE INVENTION

Conventional gyricon displays, as described in U.S. Pat. Nos. 4,126,854,4,143,103, 5,604,027, 5,717,514, 5,894,367, 5,739,801 and 6,055,091,each incorporated herein by reference in its entirety, require a sourceof electrical power. Gyricon materials respond to the application ofhigh electric fields. However, the electric current requirements can bevery low. Basically, the energy required to change the state of thedisplay is the same as that needed to charge the capacitance of thedisplay structure.

As a result, according to this invention, triboelectrically generatedcharges can be used to write, re-write or erase a gyricon sheet.

This invention provides a gyricon sheet encapsulated between aconducting plate, and a thin, insulating film.

The invention separately provides a gyricon display addressable by atriboelectric charge.

The invention separately provides for erasing an image formed on agyricon display using a tribo-electric charge.

The invention separately provides a gyricon display that does notrequire an external electric power source to form or erase an image fromthe gyricon display.

The invention separately provides a gyricon display having a substratethat is sufficiently conductive to dissipate the small amounts of chargegenerated by tribo-electric effects.

According to one exemplary embodiment of the electric paper of thisinvention, a tribo-electric charge induced on the surface of aninsulating film generates a sufficient electric field to change thestate of one or more gyricon rotatable elements of the electric paper.

According to a second exemplary embodiment of the electric paper of thisinvention, the tribo-electrically addressable electric paper can be usedto form a whiteboard that does not require chalk, solvent based pens or,in fact, any type of pen.

In the exemplary embodiment of the tribo-electrically addressableelectric paper of this invention, sufficient electric energy to producean image is provided by tribo-electric charges stored and dissipated ona surface of the display.

Gyricon displays designed to be addressed by a stylus can sometimes beadversely affected by tribo-electric effects. For example, in the caseof gyricon displays typically addressed by an electrically drivenstylus, inadvertently contacting the gyricon sheet with the user'sfinger rather than with the writing stylus can, under certainconditions, cause rotation of the gyricon rotatable elements due totribo-electric effects.

In another exemplary embodiment of the electric paper of this invention,the elastomer layer of the electric paper is sufficiently conductive todissipate small amounts of tribo-electrically generated charge. That is,the elastomer layer effectively discharges the tribo electricallygenerated charge in a time that is less than that required to causerotatable element rotation. Intentionally applied voltage, as from apowered voltage source such as a writing stylus, has an effectivelyinfinite supply of charge and will deliver a sustained voltage until thedesired writing occurs.

In another embodiment, a gyricon display is provided which can beaddressed by depositing an addressing charge on the surface of thedisplay but which dissipates the charge over a small period of timewhich provides for safe handling of the display after a short period oftime without experiencing image destructive tribo-electric effects.

BRIEF DESCRIPTION OF THE DRAWINGS

The exemplary embodiments of the invention will be described in detail,with reference to the following figures in which:

FIG. 1 shows a cross-section of one exemplary embodiment of atriboelectrically addressable display; and

FIG. 2 shows a cross-section of one exemplary embodiment of atriboelectrically suppressed gyricon display.

FIG. 3 shows a circuit model diagram of a gyricon device.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a cross-sectional view of one exemplary embodiment of asheet 100 of tribo-electrically addressable electric paper according tothe invention. Specifically, as shown in FIG. 1, a conductive basesubstrate 130 forms the base substrate of the sheet 100 oftribo-electrically addressable electric paper. Although, this embodimentshows the conductive base substrate 130 to be made integral with thesheet 100, it should be noted that it need not be so. The sheet 100 canbe manufactured without the conductive base substrate 130 which can belater provided in the form of a conductive backplane. However, when thegyricon sheet 100 is made with a conductive base substrate 130, then onesurface of a gyricon substrate 120 is substantially contiguous with theconductive base substrate 130. The gyricon substrate 120 includesgyricon rotatable elements 200 disposed within the substrate 120. Eachgyricon rotatable element 200 has two distinct portions 220 and 230, oneblack and the other white, and each portion 220 and 230 has a distinctelectrical characteristic, i.e., a zeta potential with respect to adielectric fluid, so that the gyricon rotatable elements 200 areelectrically as well as optically anisotropic.

The gyricon rotatable elements 200 are embedded in a sheet of opticallytransparent material, such as an elastomer layer, that makes up thegyricon substrate 120. The gyricon substrate 120 also contains amultiplicity of cavities 122 and is permeated by a transparentdielectric fluid, such as a plasticizer. The fluid-filled cavities 122accommodate the gyricon rotatable elements 220. In particular, there isone gyricon rotatable element 200 per cavity 122 in the gyriconsubstrate 120. The cavities prevent the gyricon rotatable elements 200from migrating within the gyricon substrate 120. Each gyricon rotatableelement 200 can be selectively rotated within its respectivefluid-filled cavity 122 by applying an electric field to present eitherthe black portion 230 or the white portion 220 to an observer viewingthe surface of the sheet 100. Thus, applying an electrical field that isadjustable in two dimensions causes the black and white portions 230 or220 of the gyricon rotatable elements 200 to appear as image elements,i.e., subpixels or pixels of a display.

An insulative layer 110 is substantially contiguous with the othersurface of the gyricon substrate 120. The insulative layer 110 may beany polyester or plastic material or any other known or later developedtransparent material that is sufficiently insulative thattribo-electrically generated charges persist for a time sufficient tocause the gyricon rotatable elements 220 to rotate. The insulative layer110 is preferably uncoated. Critical features of the insulative layer110 include its bulk resistivity and its dielectric constant. Bulkresistivity refers to the ability of the material to resist a flow of acharge across its bulk and to hold a voltage. The dielectric constant ofthe layer is proportional to the capacitance of the layer. Theinsulative layer 110 is transparent, but if it is not transparent, theconductive base substrate 130 will be transparent.

As shown in FIG. 1, the gyricon rotatable element 200 has two portions220 and 230. A segment line 210 divides the gyricon rotatable element200 into two separate sections. The first portion 220 is made using awhite pigment. The second portion 230 is made using a black pigment. Thegyricon rotatable element 200 can thus either display a white or blackface depending on its orientation with respect to a surface of the sheet100. Although only two gyricon rotatable elements 200 are shown, itshould be understood that the gyricon substrate 120 may include a verylarge number of gyricon rotatable elements 200, depending on theresolution desired. Further, although the gyricon rotatable elements 200are described as having two sections, one black and one white, it shouldbe understood that each gyricon rotatable element 200 may have more thantwo portions and may be any two or more two colors, not just black andwhite. Furthermore, although the gyricon rotatable elements 200 havebeen described as substantially spherically shaped rotatable elements,they may equally well be substantially cylindrically shaped.

Tribo-electricity is of great importance to xerography, where it is usedto impart an electrical charge on the generally dielectric tonerparticles. This enables the toner particles to be attracted to theimage-wise charge on the photoconductor drum, thus developing the chargeimage. In many forms of xerography, the toner particles are given acharge by causing them to collide and rub against developer beads. Sometypes of coatings will charge the toner beads positive and othersnegative.

“Xerography and Related Processes”, by J. Dessauer et al., The FocalPress, London and New York, First Edition, 1965, pg. 270, describes thetriboelectric process in terms of the Fermi levels of the two materialsbeing rubbed together. The direction of charge transfer depends on therelative positions of the Fermi levels. A triboelectric series can beestablished by listing a variety of materials in the order of theirrelative Fermi energies. If the window material of thetribo-electrically addressable electric paper 100 is made of a givenmaterial, for example, Mylar, and if it is rubbed by a material having aFermi energy above that of Mylar, that material will donate electrons tothe Mylar, charging the Mylar negative. On the other hand, if the Mylaris rubbed by a material having a Fermi energy below that of Mylar, theMylar will donate electrons to it, leaving the Mylar with a positivecharge.

In practice the Fermi energies of insulators are difficult to determineand trial and error is the best method of determining the triboelectricseries specific to the chosen window material. According to thisembodiment, the window material is rubbed with a trial stylus materialand the resultant polarity and magnitude of charge on the windowmaterial is measured with an electrostatic voltmeter.

Dessauer further describes that after two bodies are rubbed together,producing triboelectric charges, it is necessary that at least one ofthe bodies be a good insulator, thereby preventing the triboelectricallygenerated charges on one body from recombining with the oppositepolarity charges on the other body before the bodies can be effectivelyseparated. The required resistivity is placed at about 10⁹ ohm-cm. Theother body can be a conductor. According to this embodiment, it isnecessary that the insulative layer 110 have good insulative propertiesso it will not act to shield the gyricon material from the effects ofthe triboelectric charge.

In operation, a tribo-electric charge may be applied at the surface ofthe insulative layer 110, for example, at a point 140 in FIG. 1, by ahuman finger. In this case, a user seeking to generate a display dragshis or her finger on the surface of the insulative layer 110. The user'sfinger acts as a stylus. The user moves his or her finger in the patternof the display that is desired. The action of dragging a finger on thetransparent insulative layer 110 leaves a charge trail that activatesthe gyricon rotatable elements 200, causing them to rotate one portiontoward the surface of the transparent insulative layer 110.

In particular, the tribo-electrically generated charge trail is storedin or on the transparent insulative layer 110 and creates an electricfield substantially directed into the gyricon substrate 120. Theelectric field, generated by the tribo-electrically generated chargesstored in or on the transparent insulative layer 110, causes the gyriconrotatable elements 200 to rotate to a particular orientation. Generally,the tribo-electric charge generated by the user moving his finger overthe transparent insulative layer 110 causes the gyricon rotatableelements 200 to rotate so that the black portion faces the transparentinsulative layer 110, so that an observer sees a black, i.e., filled-in,pixel. This tribo-electrically generated charge is held for a shortwhile but the image storage properties of the gyricon act so that theblack portion faces the surface of the display device until it isdisrupted by a subsequent opposite electric field.

The display can be erased by using an “eraser” that applies a chargeopposite to the tribo-electric charge applied by the user to address thedisplay. Such an “eraser” generates a tribo-electric charge in or on theinsulative layer 110 that has an opposite polarity to the chargegenerated by the user. The action of dragging the “eraser” on theinsulative layer 110 leaves a charge trail that has the oppositepolarity to the charge trail of the user to activate the gyriconrotatable elements 200, causing the gyricon rotatable elements 200 torotate the opposite portion toward the surface of the insulative layer.

In particular, the tribo-electrically generated charge trail is storedin or on the surface of the insulative layer 110 and induces an electricfield opposite the electric field generated by the stylus or the fingerof the user. This electric field generated by the charges stored in thetransparent layer 110 causes the gyricon rotatable elements 200 torotate to a particular orientation. Generally, the tribo-electric chargegenerated by the eraser causes the gyricon rotatable elements 200 torotate so that the white portion faces the insulative layer 110, so thatan observer sees a white, i.e., blank or empty, pixel.

Thus, the electrical properties of the transparent insulative layer 110are important for the functioning of the tribo-electrically addressabledisplay. Specifically, the insulative layer 110 must be sufficientlyinsulative to hold the tribo-electric charge applied by the user or theeraser and it must have sufficient capacitance to store this charge forenough time for the rotatable elements to rotate, in spite of theleakage of charge caused by the finite conductivity of the layer 110.The bulk resistivity ρ and the dielectric constant ε are important. Theproduct of the bulk resistivity and the dielectric constant (ρε) is thetime constant T. The time constant T of a material corresponds to therate of charge dissipation in that material. Generally, given the bulkresistivity, the dielectric constant and the time constant, the rate ofcharge dissipation follows the exponential formula e^(−t/T), where t isthe time elapsed.

In general, to successfully address the tribo-electrically addressablesheet 100 of the gyricon display according to the invention, theinsulative layer 110 must be able to hold a sufficient charge for asufficient amount of time to allow an electric field, sufficient tocause the gyricon rotatable elements 200 to rotate, to persist in thesheet 100. Thus, there is a maximum rate of dissipation above which thesheet 100 will not be able to be addressed. The relationship can beexpressed as:

ρε>T _(address, max)  (1)

where:

ρ is the bulk resistivity of the film;

ε is the dielectric constant of the film; and

T_(address, max) is the time needed for complete rotation of allrotatable elements, typically 100 milliseconds.

Thus, as shown by the relationship above, the product ρε, or the timeconstant T, of the material used to form the insulative layer 110 mustexceed the worst case time constant T_(address, max) i.e., the rate ofcharge dissipation must be less than the minimum dissipation that stillallows rotation of the rotatable elements.

The tribo-electrically addressable sheet 100 can be used to form adisplay according to this embodiment that can be manufactured in manydifferent configurations. For example, the tribo-electricallyaddressable sheet 100 can be made in the size of a standard notepad,which can then be used like a conventional sheet of paper allowing easytransportability. Alternatively, the tribo-electrically addressablesheet 100 can be constructed in the size of a scribble or chalk board. Atribo-electrically addressable scribble board can have adhesive backingallowing the display to be connected to a surface, such as, for example,a door or wall. The advantages of a trib-oelectrically addressablescribble board include the ability to address the board using the touchof a finger, rather than requiring a pen, marker or chalk, which caneasily be lost.

According to another exemplary embodiment of the invention, a gyricondisplay is provided which is addressed by a conductive stylus connectedto a power supply but is not affected by stray or excessivetribo-electric charges, such as caused by handling. The gyricon displayaccording to this exemplary embodiment is referred to as atribo-electrically suppressed display.

FIG. 2 is a cross-sectional view of one exemplary embodiment of a sheet300 of a tribo-electrically suppressed electric paper according to theinvention. Specifically, FIG. 2 shows a tribo-electric displayaddressable using a stylus that includes a conductive base substrate 330that forms the base substrate of the sheet 300 of the tribo-electricallysuppressed electric paper. Again, although this embodiment shows theconductive base substrate 330 being integral with the sheet 300, thisneed not be so. The sheet 300 can be manufactured without the conductivebase substrate 330 which can be provided later as a conductivebackplane.

One surface of a gyricon substrate 320 is substantially contiguous withthe conductive base substrate 330. The gyricon substrate 320 includesthe previously-described gyricon rotatable elements 200 disposed withinthe substrate.

As described in connection with the previous embodiment, each gyriconrotatable element 200 is both electrically and optically anisotropic.For instance the gyricon rotatable elements 200 may be sphericallyshaped and have two portions 220 and 230, one black and the other white,and each portion 220 and 230 having a distinct electricalcharacteristic, i.e., a zeta potential with respect to a dielectricfluid, so that the gyricon rotatable elements 200 are electrically aswell as optically anisotropic. The gyricon rotatable elements 200 areembedded in a sheet of optically transparent material, such as anelastomer layer, that makes up the gyricon substrate 320.

A substrate 310 having a structure of conductive islands 350 withchannels 360 therebetween on its outer surface is substantiallycontiguous with the other surface of the gyricon substrate 320. Thesubstrate 310 may be any polyester or plastic material or any otherknown or later developed material that is sufficiently conductive suchthat tribo electric charges will be conducted away in a short enoughperiod of time to prevent rotation of the gyricon rotatable elements200. The conductive islands may be any material, such as indium tinoxide, that is sufficiently conductive that the stylus moving acrossthem at normal writing speed will be able to deposit enough charge onthem to nearly or completely raise their voltages to that of the stylus.Typically, this means that the conductive layer should have a sheetresistivity less than a few tens of thousand of ohms per square. Asdescribed above in connection with the tribo-electrically addressableelectric paper, critical features of the substrate 310 includes its bulkresistivity and its dielectric constant, since these determine how longan individual island retains the charge given to it by the stylus.

Again, either the substrate 310 or the substrate 330 must be made to besufficiently transparent to provide an image viewing surface for thesheet 300.

In conventionally addressable displays, which are addressed by a stylus340, contact of the gyricon display with a user's finger rather thanwith the stylus can inadvertently cause rotation of the gyriconrotatable elements 200. There are two causes of these unwanted effects.The first of these is the tribo-electric deposition of charge on theconductive islands 350 which causes electric fields to manifest andresults in inadvertent image changes. Any deposition of charge on theconductive islands 350, either intentional or not, results in imagecharges accumulating in the elastomer of the gyricon substrate 320 aftersome period of time. The second of the unwanted affects is caused by thepresence of these accumulated image charges. These persisting imagecharges can cause undesired electric fields to manifest after a changein potential on the conductive islands 350, such as is caused by contactwith a finger or other slightly conducting object, and result ininadvertent image changes. This display greatly reduces this problem byselecting an appropriate bulk resistivity for the gyricon substrate 320and the substrate 310. This allows any deposited triboelectric charge,which is limited in magnitude and occurs over a very short period oftime, to be conducted away in a short time compared with the minimumaddress time, thus causing no rotatable element 200 rotation. Since thestylus is connected to a power supply it can sustain the voltagerequired for rotatable element 200 rotation for a long enough time tocause the rotatable element 200 to complete rotation.

The following relationship illustrates the relation between the timeconstant of the elastomer part of the gyricon layer 320 and thesubstrate 310 and the minimum rate of charge dissipation needed tosuppress unwanted rotatable element rotation:

ρε<T _(address, min)  (2)

where:

ρ is the bulk resistivity of the elastomer;

ε is the dielectric constant of the elastomer; and

T_(address, min) is the minimum time for which some rotatable elementrotation is possible, which can be as high as 100 msec.

Thus, the relationship described above shows that for a given materialused to form the transparent conductive layer 310 and the gyriconsubstrate 320, the product of ρε must be less than or equal to theminimum rate of charge dissipation 10 which discharges straytribo-electric charges. If this is accomplished, stray tribo-electriccharges are effectively discharged, while the display can still beaddressed by the stylus 340.

The elastomer, which may be a material such as Dow Coming Sylgard 184,has a bulk resistivity in excess of 10¹⁶ ohm centimeters. This can belowered for the requirements of the invention by the addition ofchemicals or dopants that promote conductivity, such as fatty acidsalts, exemplified by aluminum stearate. These chemicals may be added tothe uncured elastomer. Likewise, the substrate 310 can be altered as alow conductivity polymer with a similar time constant as the suitablymodified elastomer.

Up to this point the embodiments described have used a gyricon substrate320 and substrate 310 which have similar time constants or similar bulkresistivities and dielectric constants. Alternatively, the substrate 310and conductive islands 350 can be designed to retain enough charge onthe transparent conductive island 350 for a sufficient period of time tofacilitate an image change and then to drain the charge from thetransparent conductive island 350. This requires that the substrate 320and substrate 310 have differing time constants. This allows for fasteraddressing speeds because the addressing mechanism does not need to stayin contact with a particular transparent conductive island 350 for theentire time to cause rotation of the gyricon rotatable elements 200associated with that transparent conductive island 350, but can insteaddeposit the requisite charge and move on. This example must be analyzedin terms of the physical geometries due to the differing time constantsof the elastomer and the substrate 310.

One method to do this is to use a conductive material for the channels360. By controlling the conductivity of the channels 360, or converselythe resistivity of the channels 360, a sufficient charge can bemaintained on the conductive islands 350 to insure an image change whileallowing charge to leak from the conductive islands 350. When asufficient amount of charge has leaked from the conductive islands 350then a sudden change in potential will not cause an image change. As thechannels 360 are made up of portions of the substrate 310, this meansthe resistivity of the substrate 310 must be controlled.

In order to effect an image change by rotating the gyricon rotatableelements 200, the charge density on the conductive islands 350 mustremain either relatively stable or above a threshold amount for a periodof time sufficient to cause an image change. This is so that asufficient electric field will be applied for a sufficient length oftime to cause rotation of the gyricon rotatable elements 200.

In order to avoid unwanted rotations of the gyricon rotatable elements200 when the potential is reduced or set to zero on the conductiveislands 350, either the charge must removed at a relatively slow rate orthe charge density must be less than is needed to cause an elementrotation when the potential is reduced or set to zero. This is to avoidundesirable fields created by mobile charges in cavity 122 which hadrearranged in response to the charge placed on conductive island 350.

The sequence of events is therefore as follows. First a charge is placedon a conductive island 350 to create a more than sufficient electricfield to cause an image change at time T₁. However, because the channels360 are conductive, charge will immediately start to leak from theconductive islands 350. This is why an excess of charge must be placedon the conductive island 350 so that, in the time it takes the new imageto form, until time T₂, there will still be sufficient charge to createa sufficient field to cause an image change. The time between T₁ and T₂can be as large as 100 milliseconds.

At time T₂, because the image has changed there is no longer a need fora minimum level of charge to remain on the conductive islands 350 andthe charge density continues to drop as charge continues to leak fromthe conductive islands 350. However, the sheet should not be handled ortouched with a grounded object because sufficient charge still exists onthe conductive islands 350 to cause an image change during a suddenchange in potential on conductive island 350 such as by a sudden chargeremoval.

However at time T₃ the charge remaining on conductive islands 350 hasdropped to sufficient levels that the sheet is safe to handle. We canset the difference between times T₂ and T₃ to be some reasonable number,for instance, we might consider a range of from 1 to 5 secondsacceptable.

Consider the circuit model 400 shown in FIG. 3. The circuit model 400 isa circuit model of the capacitances and resistances in a sheet 300.Resistor R1 represents the resistance in the substrate 310 of the sheet300. Capacitor C1 represents the capacitance of the substrate 320between the substrate 310 and the cavity 122. Capacitor C2 representsthe capacitance of the substrate 320 between the cavity 122 and thelayer 330. Resistor R2 represents the equivalent resistance of theparallel resistances across all the cavities 122 underneath atransparent conductive island 350. If there is only one cavity thenresistor R2 would represent the resistance across that single cavity122. If there are more than one cavity, as is likely, then resistor R2would represent the addition of all the parallel resistances, keeping inmind that when parallel resistors are added that the equivalentresistance is less than the smallest individual resistance. Therefore asthe number of cavities 122 underneath the transparent conductive island350 increases, the resistor R2 decreases.

The value of resistor R2 for a single cavity can be modeled with thecharge mobility and the electric field needed to move the charge toproduce and equation:

R 2=d ² /Qμ  (3)

Where d is the diameter of the cavity,

Q is the charge in the cavity, and

μ is the charge mobility of the charges in the cavity.

The value of resistor R1 can be modeled using the sheet resistance ofthe layer 310 and a small constant which is the ratio of the size of thesheet to the size of the transparent conductive island to produce anequation:

R 1≈(ρ_(□)/2π)I _(n)(w/r _(o))  (4)

Where:

ρ_(□) is the sheet resistance of layer 310,

w is the width of the sheet and,

r_(o) is the radius of a exemplary conductive island 350.

We know that in order to avoid the unwanted tribo-electric effects thatresistor R1 should be larger than resistor R2 to ensure that thepotential developed across resistor R2 will always be a small fractionof the applied potential.

We can estimate the charge in a single cavity 122 using Gauss' law andexperimental data about the electric field needed to rotate a gyriconrotational element 200 in a cavity 122 to obtain:

Q=επ(d ²/4)E  (5)

Where:

ε is the dielectric constant

d is the diameter of the cavity 122, and

E is the electric field needed to rotate an gyricon rotational element200.

Substituting equation (5) into equation (3) and using the followingvalues:

Number of cavities 122 per transparent conductive island 200=6.25,

μ≈10⁻⁹

ε=2ε₀

E=2×10⁵

we obtain a value for resistor R2 equal to approximately 5.75×10¹³ ohmsor a sheet resistance of layer 310 of 2.62×10¹⁴ ohms per square.

We can estimate the value of C1 and C2 at approximately 5×10⁻¹⁵ farads,and we know that the time constant of the circuit is approximately theproduct of resistor R1 and capacitor C1, that is

T=(R 1)(C 1)  (6)

Substituting the value in for C1 and letting T range from 1 to 5 secondsreturns a value for resistor R1 of from approximately 2×10¹⁴ toapproximately 1×10¹⁵ ohms or a sheet resistance of approximately9.11×10¹⁴ to 4.55×10¹⁵ ohms per square. This answer can be verified bycomparing it with our estimated value for resistor R2 at 5.75×10¹³ ohmsto verify that resistor R1 is indeed larger than resistor R2 by a factorof approximately 3.5 to 17.4.

While the value chosen for resistor R1 will vary depending on thedifferent variables discussed above such as for instance, the timeconstant, the number of cavities beneath a transparent conductiveisland, the electric field used to rotate a gyricon rotatable element,or the capacitance across a cavity, values for resistor R1 should fallin the range of at least approximately 10¹⁴ ohms to 10¹⁶ ohms usingcurrent construction techniques. If for instance a method is found ofconstructing the sheet 300 such that the resistance R2 is less, thenacceptable values for resistance R1 will be less. If for some reason itshould be desirable to construct a sheet 300 with larger values ofresistor R2, then values of resistor R1 should also be correspondinglyhigher. Similarly, if methods are used to create a sheet 300 with largeror smaller values of capacitor C1, then smaller or larger values ofresistor R1 may be appropriate.

The substrate 310 of the sheet 300 can be implemented using highperformance films such as those available from the Westlake PlasticsCompany located in Lenni, Pa.

For instance, Victrex and Ultrason E films manufactured by the WestlakePlastics Company have surfaces resistivities of 1.8×10¹⁵ and >10¹⁴ ohmsrespectively.

The invention has been described in relation to the gyricon display.However, the principles it illustrates can be equally well applied tomany other high impedance displays, such as certain liquid crystaldisplays and electrophoretic displays.

While the invention has been described in conjunction with the preferredembodiments it is described above, as evident that many altematives,modifications, and variations are apparent to those skilled in the art.Accordingly, the preferred embodiments in the invention set forth aboveare intended to be illustrative and not limiting. Various changes may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A display device comprising: a) A base substrateb) An insulative layer having a time constant T, and c) A displaysubstrate having at least one rotatable element, the at least onerotatable element having an electrical and optical anisotropy whereinthe time constant T is chosen to render the rotatable elementelectrically responsive to tribo-electric charges on the insulativelayer.
 2. A display device comprising: a) A base substrate b) Aninsulative layer having a time constant T, and c) A display substratehaving at least one rotatable element, the at least one rotatableelement having an electrical and optical anisotropy wherein theinsulative layer is chosen to render the rotatable element electricallyresponsive to tribo-electric charges on the insulative layer by storinga sufficient amount of tribo-electric charges for a sufficient period oftime to cause the rotatable element to rotate.
 3. The display of claim 2wherein the triboelectric charges are stored for at least the minimumamount of time necessary to cause a rotatable element to rotate.
 4. Thedisplay of claim 2 wherein the triboelectric charges are stored for atleast 100 milliseconds.
 5. A display device having two spaced apartsurfaces, wherein one surface is a charge deposition surface,comprising: a) a display substrate having display elements, and b) aninsulative layer on the charge deposition surface wherein the insulativelayer is chosen to render the display elements responsive totribo-electric charges on the insulative layer by storing a sufficientamount of tribo-electric charges for a sufficient period of time tocause an image change in the display substrate.
 6. The display of claim5 wherein the triboelectric charges are stored for at least the minimumamount of time necessary to cause an image change in the displaysubstrate.
 7. The display of claim 5 wherein the triboelectric chargesare stored for at least 100 milliseconds.
 8. The display of claim 5wherein the insulative layer has a time constant which is at least theminimum amount of time necessary to cause an image change in the displaysubstrate.
 9. The display of claim 5 wherein the insulative layer has atime constant which is at least 100 milliseconds.
 10. The display ofclaim 5 wherein the other surface is at least partially conductive. 11.The display of claim 10 wherein the other surface is substantiallyconductive.
 12. The display of claim 5 wherein the other surface isconductive.
 13. The display of claim 5 wherein the display substrate isa gyricon substrate.